To conserve power, it is important to reduce power losses in transistors. In a metal oxide semiconductor field effect transistor (MOSFET) device, and in particular in the class of MOSFETs known as power MOSFETs, power losses can be reduced by reducing the device's drain-to-source on-resistance (Rdson).
Split gate power MOSFETs, also known as shielded gate trench MOSFETs, utilize a greater doping concentration in the epitaxial layer to reduce Rdson. Split gate power MOSFETs incorporate trench gates that include a first electrode (e.g., polysilicon, or poly-1) that is separated from a second electrode (e.g., polysilicon, or poly-2) by an isolation layer (e.g., a differential oxide layer). From a manufacturing point of view, proper formation of the isolation layer can be challenging.
In one conventional manufacturing process, the isolation layer is grown on an exposed first polysilicon (poly-1) region at the same time that the gate oxide is formed. However, the isolation layer grows much faster on top of the poly-1 than along the sidewalls of the poly-1 region. As a result, a sharp corner is formed when the polysilicon (poly-2) for the second electrode is deposited onto the isolation layer. The sharp corner can affect the reliability of the isolation layer because of the point discharge effect, and can also increase the overlap between the gate-to-source and gate-to-drain, thus increasing Ciss (the sum of the gate-source capacitance and the gate-drain capacitance). Furthermore, because the isolation layer conforms to the profile of the underlying poly-1 region, any void or defect on the poly-1 surface will be translated into a distorted oxide profile, which may significantly reduce the poly-1/poly-2 breakdown. In addition, because the isolation layer and the gate oxide are formed simultaneously, their thicknesses are highly correlated. As a result, the isolation layer cannot be made thicker to compensate for defects or other manufacturing issues without also making the gate oxide thicker.
In another conventional manufacturing process, a sidewall oxide and the polysilicon for the first electrode are deposited in a trench. Then, the first polysilicon region is recess etched and the trench is refilled with a dielectric material that has a similar etch rate as that of the sidewall oxide. After planarization, the dielectric material and the sidewall oxide are etched back to form the isolation layer. However, it can be difficult to achieve a uniform etch rate for both the dielectric and the oxide. A difference in the etch rate could affect the uniformity of the isolation layer, which can affect the device's on-resistance as well as the input capacitance. Also, after the etch is performed, a sharp corner is formed where the material remaining in the trench meets the trench sidewalls. The sharp corner can significantly retard the gate oxide thickness, which in turn can significantly reduce gate oxide breakdown.